Cyclopentadienyl-fluorenyl based metallocene catalysts are effective catalysts in the polymerization, including homopolymerization or copolymerization of olefin monomers such as ethylene, propylene and higher olefins or other ethylenically unsaturated monomers. Such metallocenes typically have metallocene ligand structures characterized by bridged cyclopentadienyl and fluorenyl groups. An example is isopropylidene (cyclopentadienyl)(fluorenyl) zirconium dichloride. The cyclopentadienyl group or the fluorenyl group or both can be modified by the inclusion of substituent groups in the cyclopentadienyl ring or the fluorenyl group which modifies the structure of the catalyst and ultimately the characteristics of the polymers produced. Thus, olefin polymers such as polyethylene, polypropylene, which may be atactic or stereospecific such as isotactic or syndiotactic, and ethylene-higher alpha olefin co-polymers such as ethylene propylene copolymers, can be produced under various polymerization conditions and employing various polymerization catalysts.
The metallocene catalysts based upon a bridged cyclopentadienylfluorenyl ligand structure can be produced by the reaction of 6,6-dimethyl fulvene, which may be substituted or unsubstituted with fluorene, which in turn may be substituted or unsubstituted, to produce the bridged isopropylidene cyclopentadienyl-fluorenyl ligand structure. This ligand is, in turn, reacted with a transition metal halide such as zirconium tetrachloride to produce the bridged zirconium dichloride compound.
Fluorenyl ligand may be characterized by a numbering scheme for the fluorenyl ligand in which the number 9 indicates the bridgehead carbon atom. The remaining carbon atoms available to accept substituents are indicated by numbers 1-4 for one C6 ring of the fluorenyl ligand, and by numbers 5-8 for the other C6 ring of the fluorenyl ligand. The cyclopentadienyl group produced by the 6,6 dimethy fulvene may be characterized by a numbering scheme in which 1 designates the bridge head carbon atom, with numbers 2 and 5 designating the proximal carbon atoms and 3 and 4 the distal atoms.
Alpha olefin homopolymers or copolymers may be produced using metallocene catalysts under various conditions in polymerization reactors which may be batch type reactors or continuous reactors. Continuous polymerization reactors typically take the form of loop-type reactors in which the monomer stream is continuously introduced into the reactor and a polymer product is continuously withdrawn. For example, polymers such as polypropylene, polyethylene or ethylene-propylene copolymers involve the introduction of a monomer stream into the continuous loop-type reactor along with an appropriate catalyst system to produce the desired olefin homopolymer or copolymer. The resulting polymer is withdrawn from the loop-type reactor in the form of a “fluff” which is then processed to produce the polymer as a raw material in particulate form as pellets or granules. In the case of C3+ alpha olefins, such as propylene, 1-butene, 4-methyl-1 pentene, 1-hexene, 1-octene, or substituted ethylenically unsaturated monomers such as styrene or vinyl chloride, the resulting polymer product may be characterized in terms of stereoregularity, for example, isotactic polypropylene or syndiotactic polypropylene.
Use structure of isotactic polypropylene can be described as one having the methyl groups attached to the tertiary carbon atoms of successive monomeric units falling on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows:
In the above formula, each vertical segment indicates a methyl group on the same side of the polymer backbone. Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad as shown above is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive pairs of methyl groups an the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.